Spark plug

ABSTRACT

A spark plug that can reduce variation in a discharge point. The spark plug includes: a center electrode; a metal shell insulating and holding the center electrode; and a ground electrode including a base material having one end portion connected to the metal shell, and a tip connected to another end portion of the base material. The tip has a discharge surface opposed to the center electrode with a spark gap therebetween. The discharge surface has a quadrangular shape and is chamfered at four sides thereof. Only a first side which is one of the four sides is provided with a C chamfer.

FIELD OF THE INVENTION

The present invention relates to a spark plug, and in particular,relates to a spark plug including a ground electrode having a basematerial with a tip joined thereto.

BACKGROUND OF THE INVENTION

Regarding a spark plug having a spark gap between a tip of a groundelectrode and a center electrode, Japanese Patent Application Laid-Open(kokai) No. 2018-156728 (Patent Document 1) discloses a configurationusing a tip having a quadrangular-shaped discharge surface.

In the conventional configuration, when a potential difference arisesbetween the ground electrode and the center electrode, an electric fieldis concentrated near sides of the discharge surface of the tip on theground electrode. Thus, discharge points (discharge occurrencepositions) on the tip are widely distributed near the four sides of thedischarge surface. When a discharge point varies around the four sides,the position of an initial flame kernel serving as a center of flamepropagation varies, and therefore there is a possibility that accuracyof combustion prediction for evaluating ignitability by the spark plugis reduced. For improving accuracy of combustion prediction, reductionof variation in a discharge point is required.

SUMMARY OF THE INVENTION

The present invention has been made to meet the above requirement, andan object of the present invention is to provide a spark plug that canreduce variation in a discharge point.

Means for Solving the Problem

To attain the above object, a spark plug of the present inventionincludes: a center electrode; a metal shell insulating and holding thecenter electrode; and a ground electrode including a base materialhaving one end portion connected to the metal shell, and a tip connectedto another end portion of the base material. The tip has a dischargesurface opposed to the center electrode with a spark gap therebetween.The discharge surface has a quadrangular shape and is chamfered at foursides thereof. Only a first side which is one of the four sides isprovided with a C chamfer.

Another spark plug of the present invention includes: a centerelectrode; a metal shell insulating and holding the center electrode;and a ground electrode including a base material having one end portionconnected to the metal shell, and a tip connected to another end portionof the base material. The tip has a discharge surface opposed to thecenter electrode with a spark gap therebetween. The discharge surfacehas a quadrangular shape and is chamfered at four sides thereof. Of thefour sides of the discharge surface, two or more sides including a firstside are provided with C chamfers. In comparison of sizes of thechamfering of the two or more sides provided with the C chamfers, thesize of the chamfering of the first side is smaller than the sizes ofthe chamfering of the other sides.

Advantageous Effects of the Invention

According to a first aspect, the four sides of the discharge surface ofthe tip are chamfered, and only the first side which is one of the foursides of the discharge surface is provided with the C chamfer.Therefore, an electric field is more concentrated near the first side,so that a discharge point is more likely to arise near the first side.Thus, variation in a discharge point can be reduced.

According to a second aspect, a size of the chamfering provided to thefirst side is smaller than sizes of the chamfering provided to the threesides other than the first side. Therefore, an electric field is evenmore concentrated near the first side. Thus, in addition to the effectof the first aspect, variation in a discharge point can be furtherreduced.

According to a third aspect, four sides of the discharge surface of thetip are chamfered. Of the four sides of the discharge surface, two ormore sides including the first side are provided with C chamfers. Incomparison of sizes of the chamfering of the two or more sides providedwith the C chamfers, the size of the chamfering of the first side issmaller than the sizes of the chamfering of the other sides. Therefore,an electric field is more concentrated near the first side. Thus, adischarge point is more likely to arise near the first side, so thatvariation in a discharge point can be reduced.

According to a fourth aspect, a size of the chamfering provided to asecond side opposite to the first side is greater than sizes of thechamfering provided to the three sides other than the second side.Therefore, an electric field is less concentrated near the second sideopposite to the first side. Thus, in addition to the effect of the thirdaspect, variation in a discharge point can be further reduced.

According to a fifth aspect, a second side opposite to the first side isprovided with an R chamfer. Therefore, a discharge point is less likelyto arise near the second side, as compared to a case where the secondside is provided with a C chamfer. Thus, in addition to the effect ofthe third or fourth aspect, variation in a discharge point can befurther reduced.

According to a sixth aspect, the first side is located closer to an endsurface of the other end portion of the ground electrode than the threesides other than the first side. An initial flame kernel arising bydischarge near the first side located closer to the end surface is lessdeprived of energy by the base material. The initial flame kernel growswell and flame propagation is readily started. Thus, in addition to theeffect of any one of the first to fifth aspects, ignitability can beimproved.

According to a seventh aspect, a melt portion for joining the tip to thebase material is formed on a back surface opposite to the dischargesurface, along the discharge surface from the end surface of the otherend portion of the base material. Near the first side of the dischargesurface, discharge occurs frequently and heat is more likely to begenerated, so that thermal stress of the tip is more likely to be great.A thickness of the melt portion in a direction perpendicular to thedischarge surface becomes smaller with increase in a distance from theend surface along the discharge surface. Therefore, thermal stress ofthe tip near the first side is more relaxed by the melt portion. Thus,in addition to the effect of the sixth aspect, breakage of the meltportion or peeling of the tip due to thermal stress can be suppressed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a half-sectional view of a spark plug according to the firstembodiment.

FIG. 2 is a plan view of a ground electrode.

FIG. 3 is a sectional view of the ground electrode along line III-III inFIG. 2.

FIG. 4 is a sectional view of the ground electrode along line IV-IV inFIG. 2.

FIG. 5 is a plan view of a ground electrode of a spark plug according tothe second embodiment.

FIG. 6 is a sectional view of the ground electrode along line VI-VI inFIG. 5.

FIG. 7 is a sectional view of the ground electrode along line VII-VII inFIG. 5.

FIG. 8 is a plan view of a ground electrode of a spark plug according tothe third embodiment.

FIG. 9 is a sectional view of the ground electrode along line IX-IX inFIG. 8.

FIG. 10 is a sectional view of the ground electrode along line X-X inFIG. 8.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, preferred embodiments of the present invention will bedescribed with reference to the accompanying drawings. FIG. 1 is ahalf-sectional view of a spark plug 10 according to the firstembodiment, with an axial line O as a boundary. In FIG. 1, the lowerside on the drawing sheet is referred to as a front side of the sparkplug 10, and the upper side on the drawing sheet is referred to as arear side of the spark plug 10. As shown in FIG. 1, the spark plug 10includes an insulator 11, a center electrode 15, a metal shell 20, and aground electrode 30.

The insulator 11 is a substantially cylindrical member made of ceramicsuch as alumina which is excellent in mechanical property and ininsulation property under high temperature. The insulator 11 has anaxial hole 12 extending along the axial line O. The insulator 11 has,substantially at the center in the axial-line direction, an annularprotruding portion 13 protruding radially outward. The insulator 11 has,on the front side with respect to the protruding portion 13, a stepportion 14 having an outer diameter that reduces toward the front sidein the axial-line direction. The center electrode 15 is provided on thefront side of the axial hole 12 of the insulator 11.

The center electrode 15 is a bar-shaped electrode held by the insulator11 along the axial line O. The center electrode 15 is formed such that acore material having excellent thermal conductivity is embedded in abase material 16. The base material 16 is formed from a metal materialmade of Ni or an alloy containing Ni as a main component. The corematerial is formed from copper or an alloy containing copper as a maincomponent. The core material may be omitted. A tip 17 containing a noblemetal is joined to a front end of the base material 16. The tip 17 maybe omitted.

The center electrode 15 is electrically connected to a metal terminal18, in the axial hole 12 of the insulator 11. The metal terminal 18 is abar-shaped member to which a high-voltage cable (not shown) isconnected, and is made of a conductive metal material (e.g., low-carbonsteel).

The metal shell 20 is a substantially cylindrical member made of aconductive metal material (e.g., low-carbon steel) and extending alongthe axial line O. The metal shell 20 includes a front end portion 21surrounding a part of the insulator 11 on the front side with respect tothe protruding portion 13, a seat portion 23 contiguous to the rear sideof the front end portion 21, a tool engagement portion 24 formed on therear side of the seat portion 23, and a rear end portion 25 contiguousto the rear side of the tool engagement portion 24. The front endportion 21 has, on the outer circumference thereof, an external thread22 formed over almost the entire length in the axial-line direction ofthe front end portion 21 and configured to be screwed into a screw holeof an engine (not shown). The front end portion 21 has, on the innercircumference thereof, a ledge portion 26 having an inner diameter thatreduces toward the front side in the axial-line direction.

The seat portion 23 is a part for restricting the screwed amount of theexternal thread 22 to the engine and applying an axial tension to thetightened external thread 22. The tool engagement portion 24 is a partwith which a tool such as a wrench is to be engaged when the externalthread 22 is screwed into the screw hole of the engine. The rear endportion 25 is an annular part bent radially inward. The rear end portion25 is located on the rear side with respect to the protruding portion 13of the insulator 11.

Between the protruding portion 13 of the insulator 11 and the rear endportion 25 of the metal shell 20, a seal portion 27 filled with powderof talc or the like is provided over the entire circumference. Anannular packing (not shown) made of metal is interposed between the stepportion 14 of the insulator 11 and the ledge portion 26 of the metalshell 20. The ground electrode 30 is connected to the front end portion21 of the metal shell 20.

The ground electrode 30 includes a base material 31 made of a conductivemetal material (e.g., Ni-based alloy), and a tip 34 joined to the basematerial 31. The base material 31 is a bar-shaped member having one endportion 32 joined to the metal shell 20 and another end portion 33 towhich the tip 34 is joined. The tip 34 has a chemical compositioncontaining one kind or two or more kinds of noble metals such as Pt, Rh,Ir, and Ru, for example. The tip 34 is joined to the base material 31via a melt portion 35. A spark gap 37 is formed between a dischargesurface 36 of the tip 34 of the ground electrode 30 and the centerelectrode 15.

The spark plug 10 is manufactured by the following method, for example.First, the center electrode 15 is placed in the axial hole 12 of theinsulator 11. Then, with electric conduction ensured between the centerelectrode 15 and the metal terminal 18, the metal terminal 18 isinserted into the axial hole 12 of the insulator 11. Next, the insulator11 is inserted into the metal shell 20 with the ground electrode 30connected thereto in advance, whereby the metal shell 20 is attached tothe insulator 11. A part from the ledge portion 26 to the rear endportion 25 of the metal shell 20 applies a compressive load in theaxial-line direction to a part from the step portion 14 to theprotruding portion 13 of the insulator 11, via the seal portion 27 andthe packing (not shown). Thus, the insulator 11 is held by the metalshell 20. Next, the base material 31 of the ground electrode 30 is bentto form the spark gap 37, thus obtaining the spark plug 10.

FIG. 2 is a plan view of the ground electrode 30. FIG. 2 shows the otherend portion 33 (see FIG. 1) of the base material 31, and the one endportion 32 (see FIG. 1) is not shown. FIG. 3 is a sectional view of theground electrode 30 along line III-III in FIG. 2. FIG. 4 is a sectionalview of the ground electrode 30 along line IV-IV in FIG. 2.

As shown in FIG. 2 to FIG. 4, the other end portion 33 (see FIG. 1) ofthe base material 31 has a first surface 38 facing the center electrode15 side, a pair of second surfaces 39 connected to the first surface 38and extending from the other end portion 33 side to the one end portion32 (see FIG. 1) side, an end surface 40 connected to the first surface38 and the second surfaces 39, and a third surface 41 connected to thesecond surfaces 39 and the end surface 40. The third surface 41 islocated opposite to the first surface 38.

The first surface 38 of the base material 31 has a recess 31 a connectedto the end surface 40 of the base material 31. The tip 34 is provided inthe recess 31 a. The melt portion 35 for joining the tip 34 to the basematerial 31 is formed on a back surface 34 a opposite to the dischargesurface 36 of the tip 34, along the discharge surface 36 from the endsurface 40 of the base material 31.

The discharge surface 36 of the tip 34 has a quadrangular shape enclosedby four sides. The discharge surface 36 is connected to side surfaces42, 43, 44, 45 of the tip 34. The side surface 42 of the tip 34 faces inthe same direction as the end surface 40 of the base material 31. Theside surfaces 43, 45 of the tip 34 respectively face in the samedirections as the second surfaces 39 of the base material 31. The sidesurface 44 of the tip 34 is located opposite to the side surface 42 ofthe tip 34. In the present embodiment, the area of the discharge surface36 of the tip 34 is larger than the area of a discharge surface 15 a(see FIG. 3) of the center electrode 15, and the entire dischargesurface 15 a of the center electrode 15 is opposed to the dischargesurface 36 of the tip 34 in the axial-line direction. The dischargesurface 15 a has a round shape.

The four sides of the discharge surface 36 are intersection linesbetween the discharge surface 36 and the side surfaces 42, 43, 44, 45 ofthe tip 34. The intersection line between the side surface 42 and thedischarge surface 36 is a first side 46. A second side 47 opposite tothe first side 46 is the intersection line between the side surface 44and the discharge surface 36. The intersection line between the sidesurface 43 and the discharge surface 36 is a third side 48. A fourthside 49 opposite to the third side 48 is the intersection line betweenthe side surface 45 and the discharge surface 36.

In the present embodiment, the first side 46 is located closer to theend surface 40 of the base material 31 than the three sides 47, 48, 49other than the first side 46. The first side 46 is almost parallel tothe end surface 40. The second side 47 is located farther from the endsurface 40 of the base material 31 than the three sides 46, 48, 49 otherthan the second side 47.

All the sides 46, 47, 48, 49 enclosing the discharge surface 36 of thetip 34 are chamfered. The discharge surface 36 is provided with Cchamfers at two or more sides including the first side 46. In thepresent embodiment, the first side 46 and the second side 47 areprovided with C chamfers, and the third side 48 and the fourth side 49are provided with R chamfers. Instead of the R chamfers provided to thethird side 48 and the fourth side 49, C chamfers may be provided to thethird side 48 and the fourth side 49.

The C chamfer provided to the first side 46 (see FIG. 3) is a cornersurface connecting the discharge surface 36 and the side surface 42. TheC chamfer provided to the second side 47 is a corner surface connectingthe discharge surface 36 and the side surface 44. Regarding the Cchamfers, the angle at which each corner surface intersects thedischarge surface 36 or the side surface 42, 44 is not limited to 45°.The angles of the corner surfaces are set to any angle greater than 0°and smaller than 90°.

A size W1 (see FIG. 3) of the chamfering provided to the first side 46is smaller than a size W2 of the chamfering provided to the second side47. The sizes W1, W2 of the chamfering of the C chamfers refer to widthsin directions that are perpendicular to the respective sides 46, 47 andparallel to the discharge surface 36.

The R chamfer provided to the third side 48 (see FIG. 4) is a roundsurface or an elliptic surface connecting the discharge surface 36 andthe side surface 43. The R chamfer provided to the fourth side 49 is around surface or an elliptic surface connecting the discharge surface 36and the side surface 45. A size W3 of the chamfering provided to thethird side 48 is almost the same as a size W4 of the chamfering providedto the fourth side 49. The sizes W3, W4 of the chamfering of the Rchamfers refer to the radii of curvature of the respective R chamfers.The sizes W3, W4 of the chamfering may be different. In the presentembodiment, the size W2 of the chamfering provided to the second side 47is greater than the sizes W1, W3, W4 of the chamfering provided to theother three sides 46, 48, 49.

The thickness of the melt portion 35 (see FIG. 3) in a directionperpendicular to the discharge surface 36 of the tip 34 becomes smallerwith increase in the distance from the end surface 40 of the basematerial 31 along the discharge surface 36, i.e., with decrease in thedistance to the one end portion 32 (see FIG. 1) of the base material 31.The thickness of the melt portion 35 at a part contacting the sidesurface 42 of the tip 34 is greater than the thickness of the meltportion 35 at a part contacting the side surface 44 of the tip 34.

The melt portion 35 is obtained by, after placing the tip 34 in therecess 31 a of the base material 31, applying a laser beam from the endsurface 40 side of the base material 31 almost in parallel to thedischarge surface 36 and scanning the laser beam from one end to anotherand of the side surface 42 of the tip 34. The laser medium may be, forexample, a fiber laser or a disk laser, but is not limited thereto. Themelt portion 35 is formed by the tip 34 and the base material 31 beingmelted with each other.

With voltage applied between the metal terminal 18 (see FIG. 1) and themetal shell 20 of the spark plug 10, when the potential differencebetween the center electrode 15 and the ground electrode 30 has reacheddischarge voltage, discharge occurs in the spark gap 37 and an initialflame kernel is formed. When the initial flame kernel has heated thesurrounding air-fuel mixture to an ignition temperature, flamepropagation begins and the air-fuel mixture is combusted.

In the ground electrode 30, an electric field is more concentrated atthe four sides of the discharge surface 36 of the tip 34, and thus adischarge point (discharge occurrence position) is likely to arise nearthe sides 46, 47, 48, 49 of the discharge surface 36. In particular, atthe sides provided with the C chamfers among the four chamfered sides46, 47, 48, 49, a discharge point is more likely to arise than at thesides provided with the R chamfers, and a discharge point is more likelyto arise at a side having a smaller chamfering size.

In the spark plug 10, the first side 46 and the second side 47 areprovided with C chamfers, and the third side 48 and the fourth side 49are provided with R chamfers. In comparison between the size W1 of thechamfering of the first side 46 and the size W2 of the chamfering of thesecond side 47 which are provided with the C chamfers, the size W1 ofthe chamfering of the first side 46 is smaller than the size W2 of thechamfering of the second side 47, so that an electric field is moreconcentrated near the first side 46. Thus, a discharge point is morelikely to arise near the first side 46, so that variation in a dischargepoint can be reduced. As a result, the initial flame kernel serving as acenter of flame propagation is more likely to be formed near the firstside 46, so that variation in the position of the initial flame kernelis reduced. Thus, accuracy of combustion prediction for evaluatingignitability by the spark plug 10 can be improved.

The size W1 of the chamfering provided to the first side 46 is smallerthan the sizes W2, W3, W4 of the chamfering provided to the other threesides 47, 48, 49. Thus, an electric field is even more concentrated nearthe first side 46, whereby variation in a discharge point can be furtherreduced.

The size W2 of the chamfering provided to the second side 47 opposite tothe first side 46 is greater than the sizes W1, W3, W4 of the chamferingprovided to the three sides 46, 48, 49 other than the second side 47.Thus, an electric field is less concentrated near the second side 47opposite to the first side 46, so that a discharge point is more likelyto arise at a part other than the second side 47 and closer to the firstside 46. Thus, variation in a discharge point can be further reduced.

The third side 48 and the fourth side 49 connecting the first side 46and the second side 47 are provided with the R chamfers. Therefore, adischarge point can be less likely to arise near the third side 48 andthe fourth side 49, as compared to a case where the third side 48 andthe fourth side 49 are provided with C chamfers. Thus, a discharge pointis more likely to arise near the first side 46, so that variation in adischarge point can be further reduced.

In the center electrode 15, an electric field is more concentrated at anedge 15 b (see FIG. 3) of the discharge surface 15 a. The entiredischarge surface 15 a is opposed to the discharge surface 36 of the tip34 in the axial-line direction, and the discharge surface 15 a has around shape. Therefore, a point where the distance from the first side46 to the edge 15 b of the discharge surface 15 a is shortest isuniquely determined on the first side 46. A discharge point is morelikely to arise near the above point on the first side 46, so thatvariation in a discharge point can be further reduced.

The first side 46 of the discharge surface 36 is located closer to theend surface 40 of the base material 31 than the other three sides 47,48, 49 of the discharge surface 36. Since a discharge point is morelikely to arise near the first side 46 having a smaller chamfering size,an initial flame kernel is more likely to be formed near the first side46. A part near the first side 46 located closer to the end surface 40of the base material 31 is more opened as compared to parts near theother sides 47, 48, 49. Therefore, an initial flame kernel arising nearthe first side 46 is less deprived of energy by the base material 31.The initial flame kernel grows well and flame propagation is readilystarted. Thus, ignitability can be improved.

On the other hand, if discharge occurs frequently near the first side46, a part near the first side 46 is more likely to generate heat, sothat thermal stress near the first side 46 of the tip 34 is more likelyto be great. The thickness of the melt portion 35 in the directionperpendicular to the discharge surface 36 becomes greater with decreasein the distance to the end surface 40 of the base material 31 along thedischarge surface 36. Therefore, thermal stress near the first side 46of the tip 34 is more relaxed by the melt portion 35. Thus, breakage ofthe melt portion 35 or peeling of the tip 34 due to thermal stress canbe suppressed.

The second embodiment will be described with reference to FIG. 5 to FIG.7. In first embodiment, the case where two or more of the four sides 46,47, 48, 49 of the discharge surface 36 of the tip 34 are provided with Cchamfers, has been described. On the other hand, in the secondembodiment, a case where only one of four sides 53, 54, 55, 56 of adischarge surface 52 of a tip 51 is provided with a C chamfer, will bedescribed. The same parts as those described in the first embodiment aredenoted by the same reference characters, and description thereof willnot be repeated below.

FIG. 5 is a plan view of a ground electrode 50 of a spark plug accordingto the second embodiment. FIG. 6 is a sectional view of the groundelectrode 50 along line VI-VI in FIG. 5. FIG. 7 is a sectional view ofthe ground electrode 50 along line VII-VII in FIG. 5. Instead of theground electrode 30 of the spark plug 10 in the first embodiment, theground electrode 50 is connected to the metal shell 20. FIG. 5 shows theother end portion 33 (see FIG. 1) of the base material 31 of the groundelectrode 50, and the one end portion 32 (see FIG. 1) is not shown.

As shown in FIG. 5 to FIG. 7, the tip 51 of the ground electrode 50 isplaced in the recess 31 a provided to the base material 31. The meltportion 35 for joining the tip 51 to the base material 31 is formed on aback surface 51 a opposite to the discharge surface 52 of the tip 51,along the discharge surface 52 from the end surface 40 of the basematerial 31.

The discharge surface 52 of the tip 51 has a quadrangular shape enclosedby four sides. The discharge surface 52 is connected to the sidesurfaces 42, 43, 44, 45 of the tip 51. In the present embodiment, thearea of the discharge surface 52 of the tip 51 is larger than the areaof the discharge surface 15 a (see FIG. 6) of the center electrode 15,and the entire discharge surface 15 a of the center electrode 15 isopposed to the discharge surface 52 of the tip 51 in the axial-linedirection.

The four sides of the discharge surface 52 are intersection linesbetween the discharge surface 52 and the side surfaces 42, 43, 44, 45 ofthe tip 51. The intersection line between the side surface 42 and thedischarge surface 52 is the first side 53. The second side 54 oppositeto the first side 53 is the intersection line between the side surface44 and the discharge surface 52. The intersection line between the sidesurface 43 and the discharge surface 52 is the third side 55. The fourthside 56 opposite to the third side 55 is the intersection line betweenthe side surface 45 and the discharge surface 52. In the presentembodiment, the first side 53 is located closer to the end surface 40 ofthe base material 31 than the three sides 54, 55, 56 other than thefirst side 53.

All the sides 53, 54, 55, 56 enclosing the discharge surface 52 arechamfered. Only the first side 53 of the discharge surface 52 isprovided with a C chamfer, and the other three sides 54, 55, 56 areprovided with R chamfers. The C chamfer provided to the first side 53(see FIG. 6) is a corner surface connecting the discharge surface 52 andthe side surface 42. An electric field is more concentrated near thefirst side 53 provided with the C chamfer, so that a discharge point ismore likely to arise near the first side 53. Thus, variation in adischarge point can be reduced.

The R chamfer provided to the second side 54 is a round surface or anelliptic surface connecting the discharge surface 52 and the sidesurface 44. Since the second side 54 opposite to the first side 53 isprovided with the R chamfer, a discharge point is less likely to arisenear the second side 54, as compared to a case where the second side 54is provided with a C chamfer. Thus, variation in a discharge point canbe further reduced.

The size W1 of the chamfering provided to the first side 53 is smallerthan the size W2 of the chamfering provided to the second side 54.Therefore, an electric field is more concentrated near the first side53, so that a discharge point is more likely to arise near the firstside 53. Thus, variation in a discharge point can be further reduced.

The R chamfer provided to the third side 55 (see FIG. 7) is a roundsurface or an elliptic surface connecting the discharge surface 52 andthe side surface 43. The R chamfer provided to the fourth side 56 is around surface or an elliptic surface connecting the discharge surface 52and the side surface 45. The size W3 of the chamfering provided to thethird side 55 is almost the same as the size W4 of the chamferingprovided to the fourth side 56. The sizes W3, W4 of the chamfering maybe different.

The size W1 of the chamfering provided to the first side 53 is smallerthan the sizes W2, W3, W4 of the chamfering provided to the other threesides 54, 55, 56. Thus, an electric field is even more concentrated nearthe first side 53, whereby variation in a discharge point can be furtherreduced.

The size W2 of the chamfering provided to the second side 54 is greaterthan the sizes W1, W3, W4 of the chamfering provided to the other threesides 53, 55, 56. Thus, an electric field is less concentrated near thesecond side 54, so that a discharge point is more likely to arise at apart other than the second side 54 and closer to the first side 53.Thus, variation in a discharge point can be further reduced.

The entire round discharge surface 15 a is opposed to the dischargesurface 52 of the tip 51 in the axial-line direction. Therefore, a pointwhere the distance from the first side 53 to the edge 15 b of thedischarge surface 15 a is shortest is uniquely determined on the firstside 53. A discharge point is more likely to arise near the above pointon the first side 53, so that variation in a discharge point can befurther reduced.

The first side 53 is located closer to the end surface 40 of the basematerial 31 than the other three sides of the discharge surface 52. Aninitial flame kernel arising near the first side 53 is less deprived ofenergy by the base material 31. Therefore, the initial flame kernelgrows well and flame propagation is readily started. Thus, ignitabilitycan be improved.

The thickness of the melt portion 35 in the direction perpendicular tothe discharge surface 52 becomes greater with decrease in the distanceto the end surface 40 of the base material 31 along the dischargesurface 52. Therefore, thermal stress near the first side 53 of the tip51 is more relaxed by the melt portion 35. Thus, breakage of the meltportion 35 or peeling of the tip 51 due to thermal stress can besuppressed.

The third embodiment will be described with reference to FIG. 8 to FIG.10. In the first embodiment, the case where opposite sides of the foursides of the discharge surface 36 of the tip 34 are provided with Cchamfers, has been described. On the other hand, in the thirdembodiment, a case where two sides sharing a vertex are provided with Cchamfers, will be described. The same parts as those described in thefirst embodiment are denoted by the same reference characters, anddescription thereof will not be repeated below.

FIG. 8 is a plan view of a ground electrode 60 of a spark plug accordingto the third embodiment. FIG. 9 is a sectional view of the groundelectrode 60 along line IX-IX in FIG. 8. FIG. 10 is a sectional view ofthe ground electrode 60 along line X-X in FIG. 8. Instead of the groundelectrode 30 of the spark plug 10 in the first embodiment, the groundelectrode 60 is connected to the metal shell 20. FIG. 8 shows the otherend portion 33 (see FIG. 1) of the base material 31 of the groundelectrode 60, and the one end portion 32 (see FIG. 1) is not shown.

As shown in FIG. 8 to FIG. 10, a tip 61 of the ground electrode 60 isplaced in the recess 31 a provided to the base material 31. The meltportion 35 for joining the tip 61 to the base material 31 is formed on aback surface 61 a opposite to a discharge surface 62 of the tip 61,along the discharge surface 62 from the end surface 40 of the basematerial 31.

The discharge surface 62 of the tip 61 has a quadrangular shape enclosedby four sides. The discharge surface 62 is connected to the sidesurfaces 42, 43, 44, 45 of the tip 61. In the present embodiment, thearea of the discharge surface 62 of the tip 61 is larger than the areaof the discharge surface 15 a (see FIG. 9) of the center electrode 15,and the entire discharge surface 15 a of the center electrode 15 isopposed to the discharge surface 62 of the tip 61 in the axial-linedirection.

The four sides of the discharge surface 62 are intersection linesbetween the discharge surface 62 and the side surfaces 42, 43, 44, 45 ofthe tip 61. The intersection line between the side surface 42 and thedischarge surface 62 is a first side 63. A second side 64 opposite tothe first side 63 is the intersection line between the side surface 44and the discharge surface 62. The intersection line between the sidesurface 43 and the discharge surface 62 is a third side 65. A fourthside 66 opposite to the third side 65 is the intersection line betweenthe side surface 45 and the discharge surface 62. In the presentembodiment, the first side 63 is located closer to the end surface 40 ofthe base material 31 than the three sides 64, 65, 66 other than thefirst side 63.

All the sides 63, 64, 65, 66 enclosing the discharge surface 62 arechamfered. Two or more sides including the first side 63, of thedischarge surface 62, are provided with C chamfers. In the presentembodiment, the first side 63 and the fourth side 66 are provided with Cchamfers, and the second side 64 and the third side 65 are provided withR chamfers. Instead of the R chamfers provided to the second side 64 andthe third side 65, C chamfers may be provided to the second side 64 andthe third side 65.

The C chamfer provided to the first side 63 (see FIG. 9) is a cornersurface connecting the discharge surface 62 and the side surface 42. TheR chamfer provided to the second side 64 is a round surface or anelliptic surface connecting the discharge surface 62 and the sidesurface 44. Since the second side 64 opposite to the first side 63 isprovided with the R chamfer, a discharge point is less likely to arisenear the second side 64, as compared to a case where the second side 64is provided with a C chamfer. Thus, a discharge point is more likely toarise at a part other than the second side 64 and closer to the firstside 63, so that variation in a discharge point can be reduced.

The R chamfer provided to the third side 65 (see FIG. 10) is a roundsurface or an elliptic surface connecting the discharge surface 62 andthe side surface 43. The C chamfer provided to the fourth side 66 is acorner surface connecting the discharge surface 62 and the side surface45. In the present embodiment, the size W3 of the chamfering provided tothe third side 65 is smaller than the size W4 of the chamfering providedto the fourth side 66. The sizes W3, W4 of the chamfering may be almostthe same or the size W3 may be greater than the size W4.

The size W1 of the chamfering of the first side 63 provided with the Cchamfer is smaller than the size W4 of the chamfering of the fourth side66 provided with the C chamfer. Therefore, an electric field is moreconcentrated near the first side 63. Thus, a discharge point is morelikely to arise near the first side 63, so that variation in a dischargepoint can be reduced.

The size W1 of the chamfering provided to the first side 63 is smallerthan the sizes W2, W3, W4 of the chamfering provided to the other threesides 64, 65, 66. Therefore, an electric field is more concentrated nearthe first side 63. Thus, a discharge point is more likely to arise nearthe first side 63, so that variation in a discharge point can be furtherreduced.

The size W2 of the chamfering provided to the second side 64 is greaterthan the sizes W1, W3, W4 of the chamfering provided to the other threesides 63, 65, 66. Therefore, an electric field is less concentrated nearthe second side 64 opposite to the first side 63, so that a dischargepoint is more likely to arise at a part other than the second side 64and closer to the first side 63. Thus, variation in a discharge pointcan be further reduced.

The entire round discharge surface 15 a is opposed to the dischargesurface 62 of the tip 61 in the axial-line direction. Therefore, a pointwhere the distance from the first side 63 to the edge 15 b of thedischarge surface 15 a is shortest is uniquely determined on the firstside 63. A discharge point is more likely to arise near the above pointon the first side 63, so that variation in a discharge point can befurther reduced.

The first side 63 is located closer to the end surface 40 of the basematerial 31 than the other three sides of the discharge surface 62. Aninitial flame kernel arising near the first side 63 is less deprived ofenergy by the base material 31. Therefore, the initial flame kernelgrows well and flame propagation is readily started. Thus, ignitabilitycan be improved.

The thickness of the melt portion 35 in the direction perpendicular tothe discharge surface 62 becomes greater with decrease in the distanceto the end surface 40 of the base material 31 along the dischargesurface 62. Therefore, thermal stress near the first side 63 of the tip61 is more relaxed by the melt portion 35. Thus, breakage of the meltportion 35 or peeling of the tip 61 due to thermal stress can besuppressed.

While the present invention has been described above with reference tothe embodiments, the present invention is not limited to the aboveembodiments at all. It can be easily understood that variousmodifications can be devised without departing from the gist of thepresent invention.

In the above embodiments, the case where the discharge surface 36, 52,62 of the tip 34, 51, 61 has a rectangular shape, has been described.However, the present invention is not necessarily limited thereto. As amatter of course, the discharge surface 36, 52, 62 may have anotherquadrangular shape. Examples of another quadrangular shape include asquare, a parallelogram, a rhombus, and a trapezoid. At least one of thefour vertices of the quadrangle may be formed to be a round surface or acorner surface, so as to remove the edge.

In the above embodiments, the case where the first side 46, 53, 63 ofthe four sides of the discharge surface 36, 52, 62 is located closest tothe end surface 40 of the base material 31, has been described. However,the present invention is not necessarily limited thereto. As a matter ofcourse, the second side 47, 54, 64 may be located closest to the endsurface 40, or the third side 48, 55, 65 may be located closest to theend surface 40. As a matter of course, the fourth side 49, 56, 66 may belocated closest to the end surface 40. That is, the first side may beany of the four sides of the discharge surface 36, 52, 62.

In the above embodiments, the case where the first side 46, 53, 63closest to the end surface 40 among the four sides of the dischargesurface 36, 52, 62 is almost parallel to the end surface 40, has beendescribed. However, the present invention is not necessarily limitedthereto. As a matter of course, the side closest to the end surface 40among the four sides of the discharge surface 36, 52, 62 may be obliqueto the end surface 40.

In the above embodiments, the case where the third side 48, 55, 65 andthe fourth side 49, 56, 66 of the discharge surface 36, 52, 62 arealmost parallel to the second surfaces 39 of the base material 31, hasbeen described. However, the present invention is not necessarilylimited thereto. The inclinations of the third side 48, 55, 65 and thefourth side 49, 56, 66 relative to the second surfaces 39 may be set asdesired.

In the above embodiments, the case where the tip 34, 51, 61 is placed inthe recess 31 a of the base material 31 of the ground electrode 30, 50,60, has been described. However, the present invention is notnecessarily limited thereto. As a matter of course, the tip 34, 51, 61may be placed and joined to the first surface 38 of the base material31, without providing the recess 31 a to the base material 31.

In the above embodiments, the case where a laser beam is applied to theend surface 40 of the base material 31 of the ground electrode 30, 50,60 to form the melt portion 35, and thereby the tip 34, 51, 61 isjoined, has been described. However, the present invention is notnecessarily limited thereto. As a matter of course, for example, a laserbeam may be applied to the second surfaces 39 of the base material 31 orthe third surface 41 of the base material 31, to form a melt portion,and thereby the tip 34, 51, 61 may be joined to the base material 31.The method for joining the tip 34, 51, 61 to the base material 31 is notlimited to laser welding. As a matter of course, the tip 34, 51, 61 maybe joined to the base material 31 by resistance welding or diffusionbonding.

In the above embodiments, the case where the discharge surface 36, 52,62 of the tip 34,51, 61 is larger than the discharge surface 15 a of thecenter electrode 15, has been described. However, the present inventionis not limited thereto. As a matter of course, the discharge surface 36,52, 62 of the tip 34, 51, 61 may be smaller than the discharge surface15 a of the center electrode 15. In this case, a part of the dischargesurface 15 a of the center electrode 15 is opposed to the dischargesurface 36, 52, 62 of the tip 34, 51, 61 in the axial-line direction.

In the third embodiment, the case where the first side 63 and the fourthside 66 of the discharge surface 62 are provided with C chamfers, hasbeen described. However, the present invention is not necessarilylimited thereto. The first side 63 and the third side 65 may be providedwith C chamfers, and the second side 64 and the fourth side 66 may beprovided with R chamfers. In addition, instead of the R chamfersprovided to the second side 64 and the fourth side 66, C chamfers may beprovided to the second side 64 and the fourth side 66.

Description of Reference Numerals

-   10: spark plug-   15: center electrode-   20: metal shell-   30, 50, 60: ground electrode-   31: base material-   32: one end portion of base material-   33: other end portion of base material-   34, 51, 61: tip-   35: melt portion-   36, 52, 62: discharge surface-   37: spark gap-   40: end surface of base material-   46, 53, 63: first side-   47, 54, 64: second side-   48, 55, 65: third side-   49, 56, 66: fourth side

1. A spark plug comprising: a center electrode; a metal shell insulatingand holding the center electrode; and a ground electrode including abase material having one end portion connected to the metal shell, and atip connected to another end portion of the base material, wherein thetip has a discharge surface opposed to the center electrode with a sparkgap therebetween, the discharge surface has a quadrangular shape and ischamfered at four sides thereof, and only a first side which is one ofthe four sides is provided with a C chamfer.
 2. The spark plug accordingto claim 1, wherein a size of the chamfering provided to the first sideis smaller than sizes of the chamfering provided to the three sidesother than the first side.
 3. A spark plug comprising: a centerelectrode; a metal shell insulating and holding the center electrode;and a ground electrode including a base material having one end portionconnected to the metal shell, and a tip connected to another end portionof the base material, wherein the tip has a discharge surface opposed tothe center electrode with a spark gap therebetween, the dischargesurface has a quadrangular shape and is chamfered at four sides thereof,of the four sides, two or more sides including a first side are providedwith C chamfers, and in comparison of sizes of the chamfering of the twoor more sides provided with the C chamfers, the size of the chamferingof the first side is smaller than the sizes of the chamfering of theother sides.
 4. The spark plug according to claim 3, wherein a size ofthe chamfering provided to a second side opposite to the first side isgreater than sizes of the chamfering provided to the three sides otherthan the second side.
 5. The spark plug according to claim 3, wherein asecond side opposite to the first side is provided with an R chamfer. 6.The spark plug according to claim 1, wherein the first side is locatedcloser to an end surface of the other end portion of the groundelectrode than the three sides other than the first side.
 7. The sparkplug according to claim 6, wherein the tip is joined to the basematerial via a melt portion, the melt portion is formed on a backsurface opposite to the discharge surface, along the discharge surfacefrom the end surface of the other end portion, and a thickness of themelt portion in a direction perpendicular to the discharge surfacebecomes smaller with increase in a distance from the end surface alongthe discharge surface.